In data transmission systems, modems and multiplex equipment partition a serial data stream into two different sequences of bit groups: namely, symbols having M bits and frames having N bits. Typically, an integral number (M/N) of frames are grouped together to form a symbol.
System synchronization at the transmitter is maintained by a symbol clock signal which identifies each symbol occurrence in the serial data stream. The symbol clock signal is incorporated into a transmitted signal by modulation of the serial data stream signal. Thus, system synchronization at a receiver is achieved simply by recovering the symbol clock signal from the modulated serial data stream.
The symbol clock signal is synchronized with each symbol occurrence. That is, each similar transition of the symbol clock signal occurs at an identical bit position within each successive symbol. Since a symbol includes an integral number of frames, the symbol clock signal is also synchronized with each group of frames. The symbol clock signal alone is employed to recover each symbol in the modem and each frame in the multiplex equipment. Hence, no ambiguity exists in using only the symbol clock signal to locate and define each frame because the symbol clock signal also coincides in phase once per symbol with an identical position of each successive integral number of frames comprising a symbol.
Known data transmission systems operate without ambiguity between the symbol clock and the frame only under the limited condition that each symbol includes an integral number of frames. A problem neither encompassed by the condition above nor addressed in the known systems occurs when a symbol includes a nonintegral number (M/N) of frames. In this situation, a so-called n-fold ambiguity arises with respect to the sequence of frames in the serial data stream because the beginning of a symbol and the beginning of a frame coincide once every n symbols or n.multidot.M bits, where n equals N/GCD(N,M) and GCD(N,M) is the greatest common divisor of the two integer variables N and M. Consequently, the symbol clock signal traverses N complete cycles every M frames. That is, the symbol clock signal which is transmitted by the modem in the modulated serial data stream coincides in phase with identical positions in successive frames only once every n symbols, i.e., the n-fold ambiguity, as opposed to once every symbol in the prior systems.
Consequently, location and identification of the frames and constituent data streams within consecutive frames is not possible in prior data transmission systems when a symbol includes a nonintegral number of frames because the signals transmitted by the modem in the modulation of the serial data stream are incapable of resolving the existing n-fold ambiguity between the symbol clock and the frame locations.